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Abstract

Transmission and distribution systems naturally operate at multiple voltage levels. The transformers connecting these levels often require on-load tap changers to regulate the voltage and maintain it within statutory limits. Conventional time grading is used between tap changer control relays when transformers are operating in series. This ensures that downstream tap changers react more slowly to voltage deviations than upstream tap changers and prevents hunting between them. This defines a fixed duty of responsibility to each tap changer. In order to improve the quality of supply it is supposed that the duty of responsibility should not be fixed and should adapt to loading conditions. The proposed system monitors both the transformer secondary voltage and the current to determine the voltage drop that would exist between consecutive transformers. The paper demonstrates that this information can be used to bias the operation of the tap changer controllers and hence improve series performance. Communications are not required between different voltage levels and therefore there is no reduction in security and extendibility of the system.